Adds description to package.json

Fixes package appearance on npmjs.org and http://addons.builtwithember.io

Currently looks like this:
![screen shot 2014-09-22 at 9 33 26 pm](https://cloud.githubusercontent.com/assets/61048/4366562/a301baf2-42c1-11e4-953e-0ae60b3b1b1d.png)